Constant velocity universal joints are roughly classified into fixed type joints which solely allow angular displacement between input and output shafts and slide type joints in which angular displacement and axial displacement are permissible, and the kind of the joint to be used is selected according to its application, condition of use, etc. As the fixed type constant velocity universal joints, Rzeppa type joints and undercut free type joints are widely known.
FIG. 7 shows, by way of example, a Rzeppa type joint 1′ (hereinafter referred to as joint 1′), which is a kind of the fixed type constant velocity universal joint. The joint 1′ is mainly composed of: an outer member 10 having at one end of a first shaft portion 11a cup portion 13 with a spherical inner surface 12, with a plurality of track grooves 14 being formed in the spherical inner surface 12 of the cup portion 13; an inner member 20 having at one end of a second shaft portion 21 an inner ring 23 with a spherical outer surface 22, with a plurality of track grooves 24 being formed in the spherical outer surface 22 of the inner ring 23; a plurality of balls 30 arranged between the track grooves 14 and 24; and a retainer 40 having a spherical outer surface 42 corresponding to the spherical inner surface 12 of the outer member 10 and a spherical inner surface 44 corresponding to the spherical outer surface 22 of the inner member 20 and equipped with a plurality of ball pockets 46 for retaining the balls 30 arranged at predetermined circumferential intervals (see, for example, JP 2003-130082 A).
In the above-described joint 1′, sphere centers O of the spherical inner surface 12 of the cup portion 13 of the outer member 10 and of the spherical outer surface 22 of the inner ring 23 of the inner member 20 substantially coincide with each other. A center O1 of the track grooves 14 of the outer member 10 and a center O2 of the track grooves 24 of the inner member 20 are axially offset in opposite directions by substantially the same distance with respect to the sphere centers O. As a result, the ball track formed by the track grooves 14 and 24 assume a wedge-like shape diverging from a depth side toward an opening side of the outer member 10. In the Rzeppa joint 1′, longitudinal sectional configurations of the track grooves 14 and 24 are curved over their entire regions, with the centers of the curved portions being the centers O1 and O2 of the track grooves 14 and 24, respectively. In contrast, in the undercut free type joint, the opening side end portion of each track groove is formed in a configuration extending straight in the axial direction.
As shown in FIG. 8, in the above-described joint 1′, when rotational torque is applied to one of the outer member 10 and the inner member 20, with the outer member 10 and the inner member 20 being at an operation angle of θ, the balls 30 are caused to reciprocate in the ball track, with the track grooves 24 of the inner ring 23 rocking relative to the track grooves 14 of the outer member 10, whereby the rotational torque is transmitted to the other member.
FIG. 9A shows an example of the above-mentioned joint 1′ as applied to a steering device 71 of an automobile. In the steering device 71, one or a plurality of intermediate shafts 75 are arranged between an input shaft 73 connected to a steering wheel 72 and a steering gear 74, and these members are connected by the joints 1′. In the steering device 71, when vibration is transmitted to the steering wheel 72 from the wheels (not shown) during traveling, there is a fear of the driver experiencing discomfort and an operational error being induced. Thus, it is necessary for the steering device 71 to be capable of preventing transmission of vibration to the steering wheel 72. Conventionally, as a means for absorbing such the vibration, there is provided between the input shaft 73 and the intermediate shafts 75 an elastic shaft coupling 76 as shown in FIGS. 9B and 9C (see, for example, JP 1996-133097 A and JP 2002-310182 A). In the elastic shaft coupling 76 of FIGS. 9B and 9C, an inner shaft 76a and an outer shaft 76b are fit-engaged with each other through the intermediation of a cushioning member 76c. 
It should be noted, however, that if not only the elastic shaft coupling 76 but also the joints 1′ can absorb vibration, it is more effective in cutting off transmission of vibration to the steering wheel 72. As shown in FIGS. 7 and 8, in the above-described joint 1′, there is provided a pressing portion 21a for axially applying an elastic pressing force to an end portion of the second shaft portion 21 constituting the inner member 20, and the retainer 40 is provided with a receiving portion 48 for receiving the pressing force from the pressing portion 21a. However, in a state where the outer member 10 and the inner member 20 are at an operation angle of θ, as shown in FIG. 8, the direction of the vibration transmitted axially from the outer member 10 and the direction in which the elastic action of the pressing member 21a is exerted differ from each other, so it is impossible to effectively absorb the vibration transmitted from the outer member 10 to the inner member 20.